1. Field of the Invention
The present invention relates generally to the field of optical fiber communications. More specifically, the invention relates to biasing an electro-optic modulator, such as a Mach-Zender modulator, for operation within the linear region of its transfer function by applying a pair of pilot tones to the modulator's input and automatically adjusting the bias point in response to the difference component of the two pilot tones produced at the modulator's output.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the areas of networking, telecommunications, and applications which rely on networking or telecommunications, there is an increasing demand for telecommunications capacity. For example, the transmission of data over a network's trunk lines (such as the trunk lines for telephone companies or for the Internet), the transmission of images or video over the Internet, the distribution of software, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically require the high speed transmission of large amounts of data, largely digital data. As applications such as the ones mentioned above become more prevalent, the demand for transmission capacity will only increase.
Optical communications systems, such as those based on optical fiber, are well-suited to meet this growing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. There is a significant installed base of fiber lines and protocols such as the OC protocol have been developed for the transmission of data over optical fibers. In addition, advances in transmitter and receiver technology have also resulted in improvements in optical communications systems, such as increased bandwidth utilization, lower cost systems, and more reliable service.
A typical optical communications system includes a transmitter, an optical channel (e.g., optical fiber), and a receiver. Within the transmitter, an electro-optic modulator is often used to modulate an optical carrier with the information to be transmitted. The modulated carrier is transmitted across the optical channel to the receiver, which retrieves the information from the carrier.
A common electro-optic modulator used in such systems is the Mach-Zender modulator (MZM), which operates on the principle of phase interferometry. The MZM, however, is an inherently non-linear device while many communications systems would benefit from linear operation. As a result, the MZM is often operated in a mode which minimizes the non-linear effects of its transfer function, such as second and higher order harmonics, by applying a bias signal to the MZM to establish an operating point, or bias point, in the most linear region of the MZM's transfer function and then operating the MZM over a limited range about this bias point.
The bias point, however, may vary due to temperature variations, signal fluctuations, manufacturing tolerances, aging, and other factors. In fact, since the MZM is based on phase interferometry, small changes in the environment or operation of the MZM may lead to significant changes in optical path lengths within the MZM which, in turn, will cause the bias point to drift significantly. If the proper bias point is not maintained, the MZM will exhibit stronger non-linearity, including the generation of even-order harmonics and the reduction of the signal strength. This, in turn, will decrease the maximum dynamic range of the optical communications link and will otherwise degrade the performance of the overall system. Therefore, it is important to control the bias signal applied to the modulator to ensure operation at the correct bias point.
In one approach to controlling the bias point, two pilot tones at different frequencies f1 and f2 are applied to the electro-optic modulator. The modulator mixes these two pilot tones producing at its output, among other terms, a component located at the sum frequency (f1+f2) of the two pilot tones. This sum component is tapped from the modulator output and used as feedback to control the bias point. The sum frequency (f1+f2), however, is often close in frequency to the second harmonics of the two pilot tones, which are located at (2 f1) and (2 f2). This imposes stringent requirements on the band-pass filter which must select the sum component while rejecting the two second harmonics. The necessarily narrow pass band of this filter further makes it difficult to obtain good signal to noise ratio for the recovered sum component. The sum component may also be at a high enough frequency to impose significant limitations on the electronics which must process the sum component.
In another approach, amplitude modulation is applied to the information signal being transmitted by the modulator. As a result, the optical output of the modulator is also amplitude modulated. This amplitude modulation at the output is detected and used to control the bias point. Amplitude modulation, however, introduces upper and lower sidebands. Since the information signal typically has a broad bandwidth, these upper and lower sidebands can also be quite wide. As a result, the amplitude modulation approach results in distortion of the information signal. An envelope detector is also required to detect the amplitude-modulated component.
Furthermore, both of these approaches rely on non-coherent detection techniques. For example, the approach based on the sum component typically relies on detecting only the amplitude, and not the phase, of the resulting sum component. The amplitude modulation approach typically relies on an envelope detector which also loses any phase information. Hence, in both approaches, control of the bias point can only be based on the amplitude and not the phase of the feedback signal, thus limiting the types and effectiveness of suitable control algorithms.
Thus, there is a need for approaches to controlling the bias point of electro-optic modulators, such as MZMs, which overcome the above drawbacks. In particular, there is a need for approaches based on coherent techniques, thus facilitating the use of control algorithms based on the phase as well as the amplitude of the feedback signal. There is also a need for approaches based on lower frequency feedback signals, thus relaxing requirements on the corresponding frequency filters and electronics. There is further a need for approaches with good signal to noise performance.